Multilevel coherent optical system

ABSTRACT

A multilevel coherent optical system, including a heterodyne transmitter and receiver, in which a multilevel signal with a coherent optical carrier is provided by modulating the phase and the polarization of the electromagnetic field propagating through a single-mode optical fiber. The transmitter comprises a coherent light source providing the optical carrier, a phase modulator modulating the phase of the carrier, a polarization modulator, and a modulation signal generator providing control signals to the phase modulator and the polarization modulator. The receiver comprises a first stage carrying out the heterodyne detection of the phase component and the phase quadrature component of the polarization of the signal received through an optical fiber, a second stage demodulating the received signal to provide the multilevel signal, and a processing circuit comparing the received multilevel signal with predetermined reference signals. Such a system exploits the four degrees of freedom of the electromagnetic field propagating through the optical fiber so as to more closely approach the theoretical Shannon limit compared with conventional systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the communication systems using opticalsignals propagating through single-mode optical fibres and, inparticular, a method of and an apparatus for generating, transmittingand receiving a multilevel optical signal.

2. Description of the Related Art

Reliable and economically competitive, coherent optical transmissionsystems which can be made available at short and medium terms allownovel network architectures to be provided regarding long-distance andhigh-performance connections and multi-user LAN (Local Area Network) andMAN (Metropolitan Area Network) connections as well. In particular, thevery large bandwidth of the single-mode optical fibres (thousands ofGHz) can be suitably exploited by providing optical FDM-systems(Frequency Division Multiplexing) in which the selection of the desiredchannel can be obtained by shifting the frequency of the localoscillator. This allows passive optical networks with very high trafficcapacity (thousands of gB/s) to be carried out. However, two importantaspects restrict on one hand the bandwidth of the single channel andlimit on the other hand the maximum number of channels which can betuned by the user. In the first instance, in fact, the main restrictionis due to the bandwidth of the photodiodes and the electronic circuits,while regarding the second instance it should be considered that thefrequency range which can be tuned by the user depends on the tunabilitycharacteristics of the laser used as local oscillator.

In order to increase the information rate of any channel, systems havebeen provided in which the information to be transmitted is coded withmore than two levels instead of being coded using only the two binarylevels as it is customary for providing a high signal receptionsensitivity. By transmitting multilevel signals an improvement ofspectrum efficiency expressed in terms of information rate per unit ofoccupied band is obtained at the cost of a reduction of the sensitivity.The known systems with two or more levels resort to the digitalamplitude and phase keying (APK) or to the digital phase shift keying(PSK) or polarization shift keying (SPSK) of the electrical component ofthe electromagnetic field associated to a coherent optical wavegenerated by a laser source.

In particular, according to the previous state of art, EP-A-0 277 427discloses methods of an devices for processing an optical signal byaltering the polarization state thereof under control of a signal at apredetermined scrambling frequency.

EP-A-0 280 075 discloses an optical low-noise superheterodyne receiverfor modulated optical signals in which a received light signal iscoupled to a coherent light signal having the same polarization. Thensuch signals are combined so as to provide two pairs of optical signals,the signal of each pair having the same polarization perpendicular tothat of the other pair, and fed to photoelements which provideelectrical signals. Such electrical signals are then summed to eachother after demodulation and after at least a phase shifting of one ofsuch signals.

In "Electronics Letters" Vol. 26, No. 4 of 15 Feb. 1990 there isdisclosed the performance of coherent optical transmission systems usingmultilevel polarization modulation based upon equipower signalconstellations at the vertices of regular polyhedra inscribed in to thePoincare's sphere.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method of generating amultilevel signal with a better performance than the known systems withregard to the signal reception sensitivity on the same number ofemployed levels. Within such general aim the invention seeks to providein particular a transmitting and a receiving apparatus carrying out theabove mentioned method.

BRIEF DESCRIPTION OF THE DRAWINGS

Such aims are achieved by the invention defined and characterized ingeneral in the claims attached to the following description in which thepresent invention is disclosed by way of a non-limitative example withreference to the accompanying drawing, in which:

FIG. 1 is a block diagram of a transmitting apparatus for a multileveloptical signal according to the present invention;

FIG. 2 is a block diagram of the detecting stage and the intermediatefrequency stage of a receiving apparatus according to the invention;

FIG. 3 is a block diagram of a multilevel signal processing stage basedon the determination of the coefficients of the inverted Jones matrix ina receiving apparatus according to the invention;

FIG. 4 is a block diagram of a multilevel signal processing stage basedupon an algorithm for providing and uptodating the values of thecomponents of the reference vectors in the receiving apparatus of theinvention;

FIG. 5 is a block diagram of the circuit of the stage of FIG. 4 foruptodating the values of the components of the reference vectors;

FIG. 6 is a diagram of the logarithm of the error probability P_(e)versus the number of the received photons per bit F for different valuesof the level number N;

FIG. 7 is a graph for the comparison of the sensitivity of the receivingapparatus (N-4Q) according to the invention, expressed in terms of thelogarithm of the number of received photons per bit F versus the levelnumber N, with the sensitivity of a N-PSK apparatus (N-level Phase ShiftKeying), a N-APK apparatus (N-level Amplitude and Phase Keying), and aN-SPSK apparatus (N-level Polarization Shift Keying with detection byStokes parameters); and

FIG. 8 is a graph for the comparison of the sensitivity of the receivingapparatus according to the invention, expressed in terms of thelogarithm of the number of received photons per bit F versus the levelnumber N, with the limit performance of the transmitting apparatusdefined by the Shannon expression of the transmitting channel capacity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrical field E(t) of an electromagnetic wave having angularfrequency ω_(o) and propagating through a single-mode optical fibre canbe written as follows:

    E(t)=E.sub.x (t)x+E.sub.y (t)y=(x.sub.1 +ix.sub.2)x+(x.sub.3 +ix.sub.4)y e.sup.iω o.sup.t

where the phase terms x₁ and x₃ and the phase quadrature terms x₂ and x₄are the components on the reference axes x and y of the polarizationstate, i.e. the vector representing the electrical field according to agiven polarization. Vector X=(x₁, x₂, x₃, x₄) can be associated to anystate of such electromagnetic field, the components of which being suchthat:

    x.sub.1.sup.2 +x.sub.2.sup.2 +x.sub.3.sup.2 +x.sub.4.sup.2 =P

where P is the transmitted optical power;

The schematic block diagram of a transmitter according to the inventionis shown in FIG. 1: a laser source 1 generates a linearly polarizedoptical carrier having a frequency, for example, of 10¹⁴ Hz, so as toform an angle of 45° with respect to the reference axes x and y. Thephase of such optical field is modulated by a phase modulator 2 with amessage, for example a voltage having a time variable amplitude α(t),which is generated by a coder 10 from a binary sequence m(t)representing an information to be transmitted. After the phasemodulation the components of the polarization state on axes x and y aresplit by a polarization selection beam splitter 3. It should be notedthat the reference axes x and y are defined by the orientation ofsplitter 3. In the upper branch the polarization of the signal isrotated by 90° by a polarization rotator 4 so as to align it with thatof the signal in the lower branch. The phase of the latter signal ismodulated by a modulator 5 with a message β(t) also generated by coder10. The two signals having the same polarization are mixed by adirectional coupler 6, the outputs of which will be as follows:

    s.sub.1 (t)=A/2 e.sup.i[ω o.sup.t+α(t)] [e.sup.iβ(t) +e.sup.iπ/2 ]

    s.sub.2 (t)=A/2 e.sup.i]ω o.sup.t+α(t)] [e.sup.iβ(t)+iπ/2 +1]

where A² is proportional to the transmitted optical power. Thepolarization state of signal s₁ is then rotated by 90° by a polarizationrotator 7' so as to make it orthogonal to that of signal s₂, the phaseof which is modulated by a modulator 8 with a message γ(t) generated bycoder 10. The resulting signals are then coupled by a polarizationselection directional coupler 9 to provide the optical signal to betransmitted through the fibre, the x and y polarization components ofwhich have the following phase terms and phase quadrature terms:##EQU1## where the function α(t), β(t) and γ(t) can have values between0 and 2π according to the selected codification method.

In particular, such functions are generated by coder 10 according to thefollowing criteria. A succession of bits representing the information tobe transmitted are fed into coder 10. Such succession is divided ingroups of bits, each group of bits representing a symbol of the alphabetused by the coder. Thus the succession of bits is transformed in asuccession of symbols. In case a N-level signal is transmitted and, forthe sake of semplicity, under the assumption that N is a power of 2,each symbol is formed by m bits where m=2 log N. Each symbol can beunivocally associated to a point of the sphere in the four-dimensionalspace in which the electromagnetic field is represented, such pointbeing determined by the vector X=(x₁, x₂, x₃, x₄) or by a tern ofgeneralized spherical coordinates α, β and γ and by the radius of thesphere, i.e. the square root of the transmitted optical power.Therefore, the transmission of a symbol corresponds to the transmissionof a well defined state of the electrical field. As the succession ofbits m(t) are fed into the coder, an association between symbols andpoints at the coordinates α, β and γ is effected; the latter are thenentered into a digital-to-analog converter and transformed to thevoltages α(t), β(t) and γ(t) which are the control signals of thedemodulators 2, 5 and 8. It should be noted that the states of theelectrical field are completely determined by the three angularcoordinates as the transmitted optical power in the apparatus of FIG. 1remains constant.

The block diagram of the stage detecting the optical signal and of theintermediate frequency stage of a receiving apparatus according to theinvention is shown in FIG. 2.

The optical signal modulated in phase and polarization and generated bya transmitter of the type shown in FIG. 1 and transmitted through asingle-mode fibre 11 is entered into a "90° optical hybrid" 13 alongwith a coherent optical signal generated by a laser source operating aslocal oscillator 12. Such signal of the local oscillator having afrequency which differs from that of the transmitted signal carrier by apredetermined amount between 10⁸ and 10⁹ Hz is linearly polarized at 45°with respect to the reference axes x and y. The 90° optical hybrid 13 isa known device having two inputs and two outputs and providing at oneoutput the sum of the input signals and at the other output the sum ofone input signal and the other input signal the phase of which isshifted by 90°. In such a case, therefore, the output signals are thephase component and the phase quadrature component of the beat signal.

The x and y components of the polarization state of the output signalsof the optical hybrid 13 are then split by polarization selection beamsplitters 14 and 15 defining with their orientations the reference axesx and y, and separately detected by four photodiodes 16, 17, 18 and 19.The four electrical intermediate frequency signals are then filtered bybandpass filters 20, 21, 22 and 23 centered about the intermediatefrequency and having a double as high bandwidth as the figure rateR_(s), i.e. the inverse of the transmission time of a symbol. A phaselocked loop (PLL) 28 and four multipliers 24, 25, 26 and 27 allow thefour intermediate frequency signals y₁, y₂, y₃ and y₄ at the outputs ofthe filters 20-23 to be translated to base band. Such signals are thenfed to four lowpass filters 29, 30, 31 and 32 having the same bandwidthas the figure rate R_(s) so as to provide four base band signals z₁, z₂,z₃ and z₄ proportional to the estimated values of the components ofvector X which are mainly impaired by the detection noise.

Two preferred embodiment of the processing apparatus have been proposedfor providing and updating the estimated values of the components ofvector X from the base band signals z₁, z₂, z₃ and z₄. Such apparatusallow the fluctuations of the polarization state of the optical signaldue to the propagation through a single-mode fibre to be compensated bymerely electronic techniques.

The operation of the first apparatus, the block diagram of which isshown in FIG. 3, is based on the determination of the inverse Jonesmatrix. As it is known, the effects due to the propagation through asingle-mode optical fibre can be taken into account by the Jones unitoperator providing the input-output relation between the polarizationstates of the optical field. Since such relation is linear, theapplication of the inverse Jones operator to the received signal allowsthe polarization state of the transmitted optical signal to bedetermined. Vector Z having the base band signals z₁, z₂, z₃ and z₄ ascomponents is multiplied in Unit 33 by the inverse Jones matrix U⁻¹ soas to provide the estimated values of the components of vector X. Thecoefficients of the matrix are determined by an algorithm based upon theconsideration that the fluctuations of the polarization state (0, 1-1Hz) due to the fibre birefringence are much slower than the figure rate(10-1000 Hz). The algorithm is implemented on the base of thecalculation of the time averages of the signals z₁, z₂, z₃ and z₄ atcoefficient units 34, 35, 36 and 37 in time intervals much longer thanthe symbol period. i.e. the transmission time of a symbol, and muchshorter than the characteristic period of the polarization fluctuations.The elements of the Jones matrix depend linearly on the averages of thesignals z₁, z₂, z₃ and z₄, as the coefficients of such linear relationare the averages of the four coordinates of the reference pointsevaluated in the set of the N feasible transmitted symbols and stored incalculation unit 38. Therefore, if the averages of the signals z₁, z₂,z₃, z₄ are known, a linear system of four equations with four unknownvalues can be implemented, the solution of which calculated incalculation unit 38 provides the real and imaginary parts of thecoefficients of the Jones matrix, the inverse of which is thencalculated in unit 33. This algorithm causes the coefficients of theJones matrix to be uptodated at the end of any time period at which thetime averages of the signals z₁, z₂, z₃ and z₄ are evaluated, thusallowing the apparatus to follow the fluctuations of the polarizationstate due to the single-mode fibre birefringence. The decision, i.e. therecognition of the state of the multilevel signal received at a giventime, is effected in comparison unit 39 by comparing the estimatedvector ε of components ε₁, ε₂, ε₃ and ε₄ with the reference vectorscorresponding to the feasible transmitted symbols, the components ofwhich have been stored in unit 39 when adjusting the apparatus. Inparticular, such comparison is effected by calculating the distancesbetween the point on the surface of the sphere in the four-dimensionalspace corresponding to the estimated vector ε and the points determinedby the reference vectors. Among the feasible transmitted symbols it isselected the symbol corresponding to the point determined by thereference vector having the shortest distance from the point ofcoordinates ε₁, ε₂, ε₃ and ε₄. The output signal of unit 30 is fed to anUser apparatus 50.

The operation of the second apparatus processing the multilevel signalis on the contrary based upon an algorithm allowing the values of thecoordinates of the reference points to be initially determined anduptodated. i.e. the components of the reference vectors on the surfaceof the sphere in the four-dimensional Euclidean space. The schematicblock diagram of such processing apparatus is shown in FIG. 4. Theapparatus determines initially the reference vectors by means of asuitable initialization sequence and subsequently effects the continuousuptodating of the components of such vectors, the values of which arefed to decision circuit 45 in which a decision is taken by the abovedescribed procedure based upon the calculation of the distance betweenthe point corresponding to the received symbol and the reference points.The decision circuit 45 in case of a N-level signal has 4N memory cellsin which the components of the N reference vectors are stored. In thetime interval between two successive uptodatings the decision circuit 45estimates the received symbol and associates it to any of the N symbolswhich can be transmitted. The uptodating of the components of anyreference vector is carried out by calculating the mean value of thevector components which are estimated by the decision circuit during theuptodating interval as corresponding to that reference vector. At theend of any uptodating interval, which is chosen also in this case muchshorter than the characteristic periods of the polarization fluctuationsand much longer than the symbol period, the reference vectors arereplaced by those corresponding to the novel components, the mean valuesof which calculated by the above described method have been stored inthe 4N memory cells.

In the diagram of FIG. 4 the uptodating operation is effected byupdating 40 formed of four circuits 41, 42, 43 and 44, each of themcomprises a switch 46 and N mean value circuits 47 for the calculationof the mean value of the signal selected by the switch. After havingestimated the received symbol, the decision circuit 45 supplies thecontrol signal formed of the components of the reference vectorcorresponding thereto to the four blocks 41, 42, 43 and 44. Such controlsignal causes any base band signal z₁, z₂, z₃ and z₄ to be enteredthrough switch 46 into circuit 47 for the calculation of the mean valuecorresponding to the reference symbol selected by the decision circuit45 among N feasible symbols which can be transmitted. Therefore, duringthe uptodating interval the outputs of the circuits 41, 42, 43 and 44supply the signals which are to be used at the uptodating time tocalculate the mean values of the components of the novel referencevectors which are then stored in the 4N memory cells of the decisioncircuit 45. The resulting processing signal of block 45 is supplied toan user apparatus 50.

The performance of the apparatus has been valued in view of thestatistics of the detection noise. In order to optimize the performance,the reference states of the transmitted optical field have been selectedsuch as to reduce to a minimum the optical power necessary to achieve apredetermined error probability. In case of a N-level signal such choiseconsists in determining the position of N reference points on the sphereof the four-dimensional Euclidean space. From an analytical point ofview the optimization of the performance can be achieved by an algorithmwhich minimizes the multi-variable function establishing therelationship between the error probability P_(e) and the coordinates ofthe N reference points. The problem cannot be analytically solved inclosed form so that a numeric algorithm has been used to minimize theabove mentioned multi-dimensional function for 3≦N≦32.

Some results regarding feasible configurations of N reference pointsobtained by the minimization algorithm of multi-variable functions andusing the downhill simplex method are shown in the following tables I,II, III, IV.

                  TABLE I                                                         ______________________________________                                        Level    φ.sup.o   ψ.sup.o                                                                           θ.sup.o                                  ______________________________________                                        1         0.00          0.00    0.00                                          2        182.65         75.52   0.00                                          3        117.70        124.54  161.56                                         4        157.16        308.49  295.89                                         5        298.07        310.91  144.30                                         ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Level                                                                         Level 1          2      3        4    5                                       ______________________________________                                        1     0.000      1.581  1.581    1.581                                                                              1.581                                   2     1.581      0.000  1.581    1.581                                                                              1.581                                   3     1.581      1.581  0.000    1.581                                                                              1.581                                   4     1.581      1.581  1.581    0.000                                                                              1.581                                   5     1.581      1.581  1.581    1.581                                                                              0.000                                   ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Level    φ.sup.o   ψ.sup.o                                                                           θ.sup.o                                  ______________________________________                                        1         0.00          0.00    0.00                                          2        180.00         0.00    0.00                                          3         57.43         90.00   0.00                                          4        113.52         2.43    90.00                                         5        212.56        270.00   0.00                                          6        122.57        270.00  180.00                                         7        211.76        332.02  270.00                                         8        327.42         90.00  180.00                                         ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        Level                                                                         Level 1      2       3    4    5     6    7    8                              ______________________________________                                        1     0.000  2.000   1.414                                                                              1.414                                                                              1.414 1.414                                                                              1.414                                                                              1.414                          2     2.000  0.000   1.414                                                                              1.414                                                                              1.414 1.414                                                                              1.414                                                                              1.414                          3     1.414  1.414   0.000                                                                              1.414                                                                              1.414 2.000                                                                              1.414                                                                              1.414                          4     1.414  1.414   1.414                                                                              0.000                                                                              1.414 1.414                                                                              2.000                                                                              1.414                          5     1.414  1.414   1.414                                                                              1.414                                                                              0.000 1.414                                                                              1.414                                                                              2.000                          6     1.414  1.414   2.000                                                                              1.414                                                                              1.414 0.000                                                                              1.414                                                                              1.414                          7     1.414  1.414   1.414                                                                              2.000                                                                              1.414 1.414                                                                              0.000                                                                              1.414                          8     1.414  1.414   1.414                                                                              1.414                                                                              2.000 1.414                                                                              1.414                                                                              0.000                          ______________________________________                                    

In particular Table I shows the values of the angular coordinates φ, Ψand θ corresponding to the points of the sphere of the four-dimensionalEuclidean space having standardized unit radius which are associated tothe reference states of the electromagnetic field in case of anoptimized five-level configuration. The angular coordinates are bound tothe components x₁, x₂, x₃ and x₄ defining the state of theelectromagnetic field by the following relations:

    x.sub.1 =cos φ cos Ψ cos θ

    x.sub.2 =cos φ cos Ψ sin θ

    x.sub.3 =cos φ sin Ψ

    x.sub.4 =sin φ

Table II shows the values of the distances between the reference pointson the sphere of standardized unit radius in case of a five-levelconfiguration; in this case the distance of any couple of points is thesame, and when that result is obtained, that is the best for simmetryreasons.

Table III shows the values of the angular coordinates φ, Ψ and θcorresponding to the points on the sphere of the four-dimensionalEuclidean space having standardized unit radius which are associated tothe states of the electromagnetic field in case of an eight-levelconfiguration.

Table IV shows the values of the distances between the reference pointson the sphere having standarized unit radius in case of an eight-levelconfiguration. In such case it was not possible to arrange the eightreference points on the four-dimensional sphere in such a way that theyare at the same distance from one another. Nevertheless the optimumconfiguration has a high simmetry as any point has six first near pointsat a distance equal to the radius of the sphere multiplied by √2 andonly one second near point at a double as high distance as the radius ofthe sphere.

In FIG. 6 the performance of the apparatus is shown by the logarithm ofthe error probability P_(e) versus the photon number per bit F for anumber N of levels equal to 4.8 and 16, respectively.

In FIG. 7 the sensitivity of the apparatus is shown by the logarithm ofthe photon number per bit versus the number N of levels at an errorprobability of 10⁻⁹. In such figure the performance of the apparatusaccording to the invention designated by N-4Q is compared with that of aN-level heterodyne PSK apparatus (N-PSK, N-Phase-Shift-Keying), aN-level heterodyne APK apparatus (N-APK, N-Amplitude-Phase-Keying), anda N-level polarization modulation apparatus with detection by Stokesparameters (N-SPSK, N-Stokes-Parameter-Shift-Keying), the former twobeing described in K. Feher "Digital MODEM Techniques", Advanced DigitalCommunications, Prentice-Hall Inc., Eaglewood Cliffs, N.J., 1987, thethird one being described in an article of S. Betti, F. Curti, G. DeMarchis, E. Iannone, "Multilevel Coherent Optical System Based On StokesParameters Modulation" which is being published on the Journal ofLightwave Technology.

In FIG. 8 the limit performance of the transmitting apparatusconditioned by the Shannon equation regarding the channel capacity isshown. The apparatus according to the invention suffers from a penaltywith respect to the Shannon limit of 8.5 dB for N=16, 7.4 dB for N=32and 7.8 dB for N=64, respectively. The performance of the apparatusaccording to the invention with respect to the compared apparatus tendsto improve as the number of levels increases as illustrated in thefollowing Table V showing the improvement in dB of the performance ofthe apparatus according to the invention with respect to that of N-SPSKand N-PSK apparatus.

                  TABLE V                                                         ______________________________________                                        N             N-SPSK   N-PSK                                                  ______________________________________                                         8            1.4       3.8                                                   16            2.3       5.4                                                   32            3.0       9.3                                                   64            3.8      10.9                                                   ______________________________________                                    

While only one embodiment of the invention has been illustrated anddescribed, it should be appreciated that several changes andmodifications can be made without parting from the scope of theinvention.

What is claimed is:
 1. A method of providing a multilevel signal on acoherent optical carrier in order to transmit information through asingle-mode optical fiber by modulating the phase and the polarizationof the carrier, comprising the steps of:coding a first, second, andthird control signal by coding a binary succession representinginformation to be transmitted and formed of a plurality of symbols eachrepresenting a predetermined state of the multilevel signal to betransmitted; modulating the phase of the carrier with the coded firstcontrol signal; dividing the modulated carrier into two signals havingthe same polarization; modulating the phase of a first of the twosignals with the coded second control signal; mixing and dividing themodulated first signal and a second of the two signals into twoorthogonal signals representing the polarization state; modulating thephase of a first of said two orthogonal signals with the coded thirdcontrol signal; and coupling the modulated first orthogonal signal and asecond of the two orthogonal signals to produce an optical signalmodulated in phase and in polarization.
 2. The method of claim 1,further comprising the step of determining the predetermined states ofthe multilevel signal to be transmitted, each represented by componentsof a four-dimensional vector defining a reference point on a surface ofa sphere of the four-dimensional Euclidean space having a radius equalto a square root of a transmitted optical mean power, by selecting therespective reference points to minimize a multi-variable functioncorrelating a bit error probability with coordinates of said referencepoints.
 3. An optical receiver for receiving a multilevel opticalsignal, comprising:a first stage including an optical local oscillatorto generate a coherent optical signal, a 90° optical hybrid to receivesaid multilevel optical signal and said coherent optical signal and tooutput a first signal, corresponding to a sum of the multilevel opticalsignal and said coherent optical signal, and to output a second signal,corresponding to a sum of the multilevel optical signal and saidcoherent optical signal with a phase of one of said multilevel opticalsignal and said coherent optical signal shifted by 90°, two beamsplitters, each to separate said first and second signals, respectively,into orthogonal polarization component signals, and four photodiodeseach to detect said separated component signals, said first stagecoupled to an optical fiber to carry out a heterodyne detection of thephase terms and the phase quadrature terms of a beat signal generatedfrom a polarized signal received by the optical fiber and the coherentoptical signal, said first stage further including four bandpass filterscentered about the intermediate frequency of the respective componentsignals detected by said four photodiodes; a second stage coupled tosaid first stage to demodulate the received signals and to provide amultilevel signal, including means for converting the intermediatefrequency signals of said four bandpass filters to respective four baseband signals; and a processing circuit coupled to said second stage tocompare said multilevel signal with predetermined reference signals. 4.The optical receiver of claim 3 for receiving a multilevel signal,wherein said processing circuit is based upon the evaluation of theinverse Jones matrix and comprises;four circuits, each for respectivelyreceiving at their inputs the four base band signals from the convertingmeans, for calculating the time averages of said signals in time periodsmuch longer than the symbol period and much shorter than thecharacteristic periods of the polarization fluctuations, and forsupplying at their respective outputs four signals representing saidtime averages; an inverse Jones matrix circuit for receiving at itsinput the four base band signals and supplying at its outputcorresponding estimated values of the transmitted multilevel signal; anda calculation circuit for receiving at its input the four signalsrepresenting the time averages of the base band signals and forcomparing said time average signals with the feasible transmittedsymbols forming the predetermined reference signals stored in thecalculation circuit to calculate the coefficients of the Jones matrixand to supply them to the inverse Jones matrix circuit; and a comparingcircuit for receiving at its input the estimated values of thetransmitted multilevel signal and for comparing said estimated valueswith the feasible transmitted symbols stored in the comparing circuit toassign to each estimated value one of the feasible transmitted symbols.5. The optical receiver of claim 3 for receiving a multilevel signal,wherein said processing circuit comprises:first circuit means forinitially determining the reference signals by an initializationsequence; and second circuit means for calculating time averages of thebase band signals in time periods much longer than the symbol period andmuch shorter than the characteristic period of the polarization statefluctuations, and for storing and updating the components of thereference signals, the first circuit means further comparing the timeaverages of the base band signals with the reference signals andassigning to each of them one of the feasible transmitted symbols, theupdating time period being much shorter than the characteristic periodof the polarization fluctuations and much longer than the symbol period.6. An optical transmitter for transmitting a multilevel optical signalmodulated in phase and polarization, to transmit information through asingle-mode optical fiber, comprising:a coder to code a first, secondand third control signal by coding a binary succession representinginformation to be transmitted and formed of a plurality of symbols eachrepresenting a predetermined state of the multilevel signal to betransmitted; a coherent light source to generate an optical carrier; afirst phase modulator, connected to said coherent light source and saidcoder, to modulate a phase of the carrier with the coded first controlsignal; a polarization selection beam splitter, connected to the firstphase modulator, to split the modulated carrier into two components ofthe polarization state of the carrier; a first polarization rotator,connected to the polarization selection beam splitter, to rotate thepolarization of a first of the two components by 90°; a second phasemodulator, connected to the polarization selection beam splitter andsaid coder, to modulate a phase of a second of said two components withthe coded second control signal; a directional first coupler, connectedto both the first polarization rotator and the second phase modulator,to superimpose the rotated first component with the modulated secondcomponent and to output a first and a second output signal,respectively; a second polarization rotator, connected to thedirectional first coupler, to rotate the polarization of the firstoutput signal by 90° so that the rotated first output signal isorthogonal to the second output signal; a third phase modulator,connected to the directional first coupler and said coupler, to modulatea phase of the second output signal with the coded third control signal;and a second coupler, connected to the second polarization rotator andthe third phase modulator, to couple the modulated second output signalwith the orthogonal first output signal to produce an optical signalmodulated in phase and polarization.
 7. An optical transmitter fortransmitting a multilevel optical signal modulated in phase andpolarization, to transmit information through a signal-mode opticalfiber, comprising:a light source to generate a carrier; a coder to codea plurality of control signals based on a binary succession ofinformation to be transmitted and a predetermined state of themultilevel signal to be transmitted; polarization and modulation meansfor modulating a phase of the carrier with one of said plurality ofcoded control signals, and for generating a first and a second signalfrom the modulated carrier, the first signal being orthogonal to thesecond signal and the second signal being further modulated with anotherone of said plurality of coded control signals; and a coupler to couplethe first and second signals to generate an optical signal modulated inphase and polarization.
 8. A method for transmitting a multileveloptical signal modulated in phase and polarization, to transmitinformation through a single-mode optical fiber, comprising the stepsof:generating a carrier; coding a plurality of control signals based ona binary succession of information to be transmitted and a predeterminedstate of the multilevel signal to be transmitted; modulating a phase ofthe carrier with one of said plurality of coded control signals;generating a first and a second signal from the modulated carrier, thefirst signal being orthogonal to the second signal and the second signalbeing further modulated with another one of said plurality of codedcontrol signals; and coupling the first and second signals to generatean optical signal modulated line in phase and polarization.